Ultrathin Piezotronic Transistors with 2-Nanometer Channel Lengths

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1 Article Subscriber access provided by Kaohsiung Medical University Ultrathin Piezotronic Transistors with -Nanometer Channel Lengths Longfei Wang, Shuhai Liu, Guoyun Gao, Yaokun Pang, Xin Yin, Xiaolong Feng, Laipan Zhu, Yu Bai, Libo Chen, Tianxiao Xiao, Xudong Wang, Yong Qin, and Zhong Lin Wang ACS Nano, Just Accepted Manuscript DOI: 0.0/acsnano.b0 Publication Date (Web): Apr Downloaded from on April, Just Accepted Just Accepted manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides Just Accepted as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. Just Accepted manuscripts appear in full in PDF format accompanied by an HTML abstract. Just Accepted manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI ). Just Accepted is an optional service offered to authors. Therefore, the Just Accepted Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the Just Accepted Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these Just Accepted manuscripts. is published by the American Chemical Society. Sixteenth Street N.W., Washington, DC 0 Published by American Chemical Society. Copyright American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

2 Page of ACS Nano 0 Ultrathin Piezotronic Transistors with -Nanometer Channel Lengths Longfei Wang,,,@ Shuhai Liu,,@ Guoyun Gao,, Yaokun Pang,, Xin Yin, & Xiaolong Feng, # Laipan Zhu,, Yu Bai,, Libo Chen,, Tianxiao Xiao,, Xudong Wang, & Yong Qin,*,, and Zhong Lin Wang*,,, CAS Center for Excellence in Nanoscience, Beijing Key Laboratory of Micro-nano Energy and Sensor, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 000, China College of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 000, P. R. China School of Advanced Materials and Nanotechnology, Xidian University, Shaanxi 00, China Institute of Nanoscience and Nanotechnology, School of Physical Science and Technology, Lanzhou University, Gansu 000, China & Department of Materials Science and Engineering, University of Wisconsin-Madison # Microsystems and Terahertz Research Center, China Academy of Engineering Physics, Chengdu, Sichuan 00, China School of Material Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United These authors contributed equally to this work. * zhong.wang@mse.gatech.edu * qinyong@xidian.edu.cn Silicon transistors rapidly approaching their scaling limit due to the short-channel effects, alternative technologies are being urgently needed for the next-generation electronics. Here, we demonstrate ultrathin ZnO piezotronic transistors with a ~-nm physical channel length using the inner crystal self-generated out-of-plane piezopotential as the gate voltage to control the carriers transport. This design removes the need for external gate electrodes that are challenging at nanometer scale. These ultrathin devices exhibit a strong piezotronic effect and excellent pressure-switching characteristics. By directly converting mechanical drives into

3 ACS Nano Page of 0 electrical control signals, ultrathin piezotronic devices could be used as active nanodevices to construct next generation of electromechanical devices for human-machine interfacing, energy harvesting and self-powered nanosystems. Keywords: piezotronic transistor, ZnO ultrathin film, piezotronics, piezoelectricity, piezotronic effect Fabrication of sub- nm silicon (Si) transistors is extremely challenging due to the weakening effectiveness of the electrostatic gate control on the channel. Transistors based on carbon nanotubes, - nanowires and two-dimensional transition metal dichalcogenides, have been explored for breaking through the size limit, but the operation of these devices still relies on the mechanism of external voltage gating. - Piezotronic transistors using inner crystal piezopotential as the gate voltage have been demonstrated for wurtzite structured semiconductors. - Such devices are innovative in a way that the traditional channel-width gating is replaced by interface gating, with a possibility of breaking through the channel width to sub-nm. However, the currently reported piezotronic transistors were made of nanowires that had sizes in the order of a few micrometers. Thus, it is unknown if such devices can be shrunk to sub-nm scale although they are gated by interface polarization charges distributed within one to two atomic layers at the metal-semiconductor interface. Piezoelectricity in bulk crystals, films and nanowires has a wide applications in transducers, energy harvesting, and electronics. - A rapid development of high performance and miniaturized electronic devices in nano-electro-mechanical systems (NEMS), calls for synthesis and discovering of low-dimensional piezoelectric materials. Many transition metal dichalcogenides (TMDs) are proposed theoretically as outstanding two-dimensional (D) in-plane piezoelectric materials., The monolayer MoS -based powering nanodevices and piezoelectric sensors have demonstrated the possible applications of D nanomaterials in nanoscale electromechanical devices. - However, the anisotropy of in-plane piezoelectricity and the great challenge to achieve large-scale directional growth of TMD monocrystal

4 Page of ACS Nano 0 seriously impede actual integration into devices.,- Recently, sub-nanometer thick platelets with wurtzite structure (Figure a) such as ZnO and CdS with polarization direction normal to the nanosheets have been synthesized. The hexagonal structure of ZnO still preserves even when its thickness is down to a few atomic layers, so that the crystal with non-centrosymmetry structure still has piezoelectric property. Simply, metal cations Zn and anions O are tetrahedral coordinated and their centers overlap with each other (Figureb and Figure c left). By applying a stress along an apex of the tetrahedron, the centers of positive and negative ions centers are displaced relatively, resulting in a piezoelectric polarization (Figure c right), which is the origin of the piezoelectricity along c-axis, which is usually the normal direction of the grown nanosheets of ZnO. Here, using the piezoelectric polarization charges presented at surfaces within one to two atomic layers thickness, ultrathin piezotronic transistors were demonstrated with a physical channel length of ~ nm, as shown in Figure d. The stress-induced strong out-of-plane piezopotential can effectively modulate charge carrier transport. This study shows the effectiveness of the gating effect of the piezopotential within an ultrashort channel length in the D wurtzite structures, with potential applications for energy harvesting, ultrasensitive sensors, and nanoscale electromechanical systems. Results and Discussion The single-crystalline ZnO ultrathin films were prepared at water-air interface. In the synthesis process, oleylsulfate monolayer was used as a template to guide the growth of ZnO nanostructure, resulting in ZnO ultrathin films at the water-air interface with c-axis perpendicular to the liquid level pointing to air (Figure S). A typical morphology of the as-obtained ZnO ultrathin films is a single triangular with edges from ~0 to µm (Figure S) and thickness of ~ nm (Figure a). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images reveal the single crystal structure of the ultrathin film (Figure S and Figure b). In the water-air growth method, an oleylsulfate monolayer was inevitably adsorbed on the ZnO ultrathin films. X-ray photoelectron spectroscopy was used to confirm the

5 ACS Nano Page of 0 presence of oleylsulfate on the surface of ZnO ultrathin film (Figure S). Given only few nanometer thickness, the molecules of oleylsulfate might have great impact on the electrical property of ZnO ultrathin film., The simulated structure of wurtzite ZnO ultrathin film and corresponding electronic band structures show that the ZnO ultrathin film is a direct bandgap p-type semiconductor. The electronic property of ZnO ultrathin film was further investigated by fabricating field effect transistors. The I d -V g curve presents a typical p-type semiconductor behavior (Figure c). Moreover, this atomic thick p-type semiconductor can be stable in the atmospheric environment, making it an ideal choice for constructing ultrathin electronic devices. For ultrathin piezoelectric devices, the piezoelectricity is the key factor that directly determines the mechanism and performance. Here, we used piezoresponse force microscopy, (PFM) to investigate the out-of-plane piezoelectricity of ZnO ultrathin film (Figure S). Conductive PFM were applied to characterize the vertical piezoresponse of ZnO ultrathin film with tip s voltages from. to. V. The amplitude images and corresponding topography and phase images under different tip voltages were presented in Figure d-i and Figure S. As expected, obvious amplitude changes were found as the tip voltage increased continuously, indicating a strong inverse piezoelectric effect. The insets are statistical distributions of amplitude values of the ZnO and the substrate. The overall amplitude changes of ZnO versus applied voltages can be derived from entire areas from Figure d-i and the calculation method were detailed in Figure S. By deriving the slope of the amplitudes versus the voltages, the effective piezoelectric coefficient (d eff ) of ZnO ultrathin film is calculated to be ~. pm V - (Figure j). It is about two times larger than that of the bulk ZnO materials, and even larger than that of some inorganic ferroelectric ceramics (Table S). The strong out-of-plane piezoelectricity of ZnO ultrathin film makes it an ideal candidate for ultrathin piezoelectric devices. According to previous studies, the piezoelectricity strongly depends on the carrier concentration of materials. For example, as the carrier concentration changed from 0 to 0 cm -, the piezoelectric coefficient of CdS will be decreased by %. Based on the dimension of the ZnO ultrathin film and the conductance, the carrier concentration was estimated to be ~0

6 Page of ACS Nano 0 cm -. Therefore, this strong out-of-plane piezoelectricity of ZnO ultrathin film is presumable due to the low carrier concentration and the changes in local polarization. We next measured the electrical transport property of ultrathin piezotronic transistors under compressive stress at room temperature. The two-terminal devices fabricated with metal-semiconductor-metal (MSM) structure (Figure a) consist of two Schottky barriers. Its conductance is dominated by the reversed metal-zno Schottky contact (Figure c and Figure S). The changed electrical transport behavior probably resulted from two effects. One is the piezotronic effect,,,- in which stress-induced piezoelectric polarization charges at metal-semiconductor interfaces asymmetrically modulate the Schottky barriers. The other is the piezoresistive effect, in which stress-induced bandgap or charge carrier density change modulate the entire resistance of the device, resulting in a symmetric volume effect without polarity. For the ultrathin piezotronic transistor, under compressive stresses, the current significantly increases under forward bias and gradually decreases under reverse bias (Figure b). This asymmetric modulation indicates that the modulation of charge carrier transport is dominated by the piezotronic effect. According to the Schottky theory, a linear relationship between ln(i) and V / or V corresponds to the presence or absence of the mirror force at the Schottky junction. By plotting both ln(i)-v / and ln(i)-v curves, we found the ln(i) is almost proportional to V / (Figure S), indicating that there exists a mirror force at the Schottky junction and the barrier is not that sharp. In addition, the changes of Schottky barrier height (SBH) calculated in Figure S0 shows an approximately linear relationship with applied pressures, further demonstrating that the stress-induced piezopotential can effectively modulate the Schottky barrier. By deriving the ln(di/dv)-v curves from the non-linear fitting of I-V characterization (Figure S), we do not find any singularity or discontinuity which would occur in a tunneling case, indicating that there is almost no tunneling current in the ultrathin ZnO piezotronic transistor. Upon applying the compressive stress, a strong piezoelectric field can be produced inside the ZnO ultrathin film because of the strong out-of-plane piezoelectricity (Figure j) and atomic thickness of ZnO (~ nm, Figure a), which not only has a

7 ACS Nano Page of 0 profound influence on the carriers concentration and distribution in ZnO ultrathin film, but also on the charge distribution in M-S interface states. 0,- Generally, the negative piezopotential can attract holes from the interface, resulting in a decreased barrier height; while the positive piezopotential can deplete holes towards the interface, resulting in an increased barrier height (Figure c). -,- The polarity of stress-induced piezoelectric polarization charges cause an opposite modulation on the barrier height of the M-S contacts, as well as the anisotropic changes in electrical transport of ultrathin piezotronic transistor. The magnitude and polarity of the piezopotential depend on the crystallographic orientation of the piezoelectric semiconductor and direction of stress. Consequently, the carrier transport across the M-S contacts can be effectively modulated by the piezoelectric polarization charges, so as to be controlled by the external stress. Hence, the mechanical stress can function as a controlling gate signal and no external gate voltage is required for a piezotronic transistor. The controllable regulation in ultrathin devices by stress-induced out-of-plane piezopotential may offer an approach for tunable electronics. To investigate the responsiveness of ultrathin piezotronic transistor to the applied mechanical stimuli, we measured the current s response to periodically switched stresses at a constant bias of.0 V (Figure d (left) and Figure S). The dynamic response indicates that the impact of compressive stress on the conductance is reversible. The measured current is immediately increased as the compressive stress increased and immediately restored to its original state as the compressive stress removed, which can be considered as on/off states in a transistor and may enable the development of next-generation logic circuits. The response is highly repeatable in many on/off cycles and no obvious degradation is observed, indicating a good repeatability and stability. The current responds very quickly to the change of stress, and the response time of ultrathin piezotronic transistor is about ~ ms (Figure d, right), which can also be enhanced by integrating local on-site signal processing circuits with an ultrathin piezotronic transistor. Next, piezotronic stress-gated OR logic gates were developed using series of ultrathin piezotronic transistors (Figure and Figure S). The forces were

8 Page of ACS Nano 0 independently applied on the respective components of the piezotronic stress-gated logic gates. Four states, 00, 0, 0 and defined here correspond to four combinations of off (0) and on () for the two equal and independently controlled forces. The measured currents for the 00 state was only ~ na under a bias of.0 V; in contrast, the measured currents were ~ na for the 0 and 0 states (Figure b, d). The charge carrier transport of an ultrathin piezotronic transistor is dominated by the reversed Schottky barrier, which is formed between top electrodes and ultrathin film. Under a single compressive force (Figure a, c), negative piezopotential induced on the top Schottky contact decreases the barrier height and hence increases the electrical transport of the device (Figure b,d), as schematically illustrated by the band diagrams (Figure c). In the state, both forces applied simultaneously, the measured current was significantly enhanced and much larger than that of 0 and 0 states (Fig. e,f). In this case, both the reverse-biased Schottky barriers were decreased by the negative piezopotential generated by ZnO ultrathin films. Taken all together, the stress-gated series of ultrathin piezotronic transistors demonstrated the spatial resolved various logic gates controlled by mechanical force, which may has applications in electronic skin, human-machine interfacing and nano-robotics. These results demonstrate that piezotronic effect is still effective at ultrathin junction, although further efforts are still required to achieve large-scale fabrication of ultrathin piezotronic transistors for practical applications. Conclusions In summary, we have successfully developed ultrathin ZnO piezotronic transistors with a ~-nm physical channel length. The charge carrier transport characteristics of our devices can be effectively gated by the piezoelectric polarization charges created at metal-semiconductor interface. The atomic thickness of ZnO and the structure of two-terminal piezotronic transistor enable the study of their physics at ultrashort channel by using the strong out-of-plane piezoelectricity, removing the need for external gate electrode or any other patterning processes that are challenging at these scale lengths. This work provides insight into the development of high performance transistor with ultrashort channel, as an alternative to conventional silicon

9 ACS Nano Page of 0 technologies. This study also proves the potential application of ultrathin piezoelectric materials in the next-generation electronics. Methods Synthesis of ZnO ultrathin films. We synthesized ZnO ultrathin films by using water-air method. In a typical preparation of ZnO ultrathin film, mm Zn(NO ) and hexamethylenetetramine (HMT) were added into ml aqueous solution. Then, 0 µl chloroform solution containing 0. vol% sodium oleyl sulfate was spread on the aqueous solution surface. Then, this solution was placed in a ºC convection oven. A single layer of ZnO ultrathin films would appear in about 00 minutes. ZnO ultrathin films morphology and band-structure characterization. For AFM, SEM, XPS characterization, ZnO ultrathin films were transferred to SiO /Si substrates. The morphology and thickness of the ZnO ultrathin films were characterized by using AFM tapping mode (MFP-D TM, Asylum Research) and A HITACHI S-0 field-emission scanning electron microscope (FE-SEM). The force constant of Si tips is. N/m, and the resonance frequency is ~ khz. Transmission electron microscopy (TEM) analysis was performed by using a FEI TECNAI F microscope. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB 0 instrument from Thermo Fisher Scientific, USA. Fabrication of FET device and electrical measurement. The ZnO ultrathin films were transferred onto the Si wafer with 0 nm thermally grown SiO. The devices were fabricated by standard e-beam lithography (EBL) process as follow: First, a layer of 0 nm positive e-beam photoresist (MICRO CHEM PMMA A) was spin-coated on the substrate at 00 rpm for s, and then baked at ºC for min. Then the source/drain electrodes were subsequently defined by standard EBL, electron-beam evaporation of Cr/Au (0 nm/ nm), and lift-off process. The electrical characterization of the devices was conducted with a semiconductor parameter analyzer (Agilent B0A) in a probe station under ambient environment. Piezoresponse force microscopy (PFM) imaging to investigate the out-of-plane piezoelectricity of ZnO ultrathin film. The out-of-plane piezoelectricity of ZnO ultrathin films was performed using AFM (MFP-D TM, Asylum Research) with PFM

10 Page of ACS Nano 0 mode. The conductive tips of Pt/Ir coating, with the force constant of. N/m, were used in PFM mode. The resonance frequency is ~0 khz for PFM mode. In order to accurately calculate the deformation of ZnO ultrathin film under different voltages, we count the amplitude values of ZnO and substrate from entire areas. Then the average amplitude variations of ZnO versus applied voltages can be calculated by subtracting the amplitude values of the substrate. To form weak indentation, the force of ~ nn was applied to the sample surface. All samples were examined in a sealed chamber under ambient laboratory conditions. Fabrication of single ultrathin piezotronic transistor and electrical measurement. Cr/Au (0 nm/ nm) was deposited on the prepared substrates as the bottom electrode through RF magnetron sputtering. Then, the ZnO ultrathin films were transferred onto the substrate with bottom electrode. The Au coated AFM plateau tips (with diameter of ~. µm and spring constant of ~. nn/nm) were used as the top electrodes of ultrathin piezotronic transistor to form M-S-M structure. The electrical transport properties of ultrathin piezotronic transistors under compressive stress were characterized by conductive AFM measurement mode. Fabrication of ultrathin piezotronic transistors and electrical measurement. The devices were fabricated by standard e-beam lithography (EBL) process as follow: Firstly, a layer of 0 nm positive e-beam photoresist (MICRO CHEM PMMA A) was spin-coated on the substrate at 00 rpm for s, and then baked at ºC for min. Then the source/drain electrodes were subsequently defined by standard EBL, electron-beam evaporation of Cr/Au (0 nm/ nm) as the bottom electrode, and lift-off process. Then, the ZnO ultrathin films were transferred onto the bottom electrodes. Finally, the top electrodes Cr/Au were prepared by standard e-beam lithography (EBL) process. The I-V characteristics of the devices were measured by a measurement system with a low-noise current preamplifier (Model No. SR0, Stanford Research Systems, Inc.) and a function generator (Model No. DS, Stanford Research Systems, Inc.). The forces were applied to the piezotronic transistors by using AFM system (MFP-DTM, Asylum Research). The measurements were taken at room temperature of ~, and the humidity is ~%.

11 ACS Nano Page 0 of 0 ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional information including Materials and Methods, Figure S to S and Table S. AUTHOR INFORMATION Corresponding Author * zhong.wang@mse.gatech.edu * qinyong@xidian.edu.cn Author L.Wang and S. Liu contributed equally to this work. The authors declare no competing financial interest. ACKNOWLEDGEMENTS We thank to Fei Wang for stimulating discussions. This research was supported by the thousands talents program for pioneer researcher and his innovation team, China, National Natural Science Foundation of China (Grant Nos.,, 0, 0, and 0), the National Key R & D Project from Minister of Science and Technology (YFA00), and the National Program for Support of Top-notch Young Professionals. REFERENCES () Franklin, A. D. Nanomaterials in Transistors: From High-Performance to Thin-Film Applications. Science,, aab. () Shulaker, M.M.; Hills, G.; Patil, N.; Wei, H.; Chen, H.; PhilipWong, H. S.; Mitra, S. Carbon Nanotube Computer. Nature,, -. () Cao, Q.; Tersoff, J.; Farmer, D. B.; Zhu, Y.; Han, S. J. Carbon Nanotube Transistors Scaled to a -Nanometer Footprint. Science,, -. () Qiu, C.; Zhang, Z.; Xiao, M.; Yang, Y.; Zhong, D.; Peng, L. M. Scaling Carbon Nanotube Complementary Transistors to -nm Gate Lengths. Science

12 Page of ACS Nano 0,, -. () Duan, X. F.; Niu, C. M.; Sahi, V.; Chen, J.; Parce, J. W.; Empedocles, S.; Goldman, J. L. High-Performance Thin-Film Transistors Using Semiconductor Nanowires and Nanoribbons. Nature 0,, -. () Zhao, M. V.; Ye, Y.; Han, Y. M.; Xia, Y.; Zhu, H. Y.; Wang, S. Q.; Wang, Y.; Muller, D. A.; Zhang, X. Large-Scale Chemical Assembly of Atomically Thin Transistors and Circuits. Nat. Nanotechnol.,, -. () Desai, S. B.; Madhvapathy, S. R.; Sachid, A. B.; Llinas, J. P.; Wang, Q.; Ahn, G. H.; Pitner, G.; Kim, M. J.; Bokor, J.; Hu, C.; Wong, H. S. P.; Javey, A. MoS Transistors with -Nanometer Gate Lengths. Science,, -0. () Wang, Z. L. Nanopiezotronics. Adv. Mater. 0,, -. () Wu, W.; Wen, X.; Wang, Z. L. Taxel-Addressable Matrix of Vertical-Nanowire Piezotronic Transistors for Active and Adaptive Tactile Imaging. Science, 0, -. (0) Wu, W.; Wei, Y.; Wang, Z. L. Strain-Gated Piezotronic Logic Nanodevices. Adv. Mater. 0,, -. () Han, W.; Zhou, Y.; Zhang, Y.; Chen, C.; Lin, L.; Wang, X.; Wang, S.; Wang, Z. L. Strain-Gated Piezotronic Transistors Based on Vertical Zinc Oxide Nanowires. ACS nano,, -. () Zhang, Y.; Yan, X.; Yang, Y.; Huang, Y.; Liao, Q.; Qi, J. Scanning Probe Study on the Piezotronic Effect in ZnO Nanomaterials and Nanodevices. Adv. Mater.,, -. () Qin, Y.; Wang, X.; Wang, Z. L. Microfibre-Nanowire Hybrid Structure for Energy Scavenging. Nature 0,, 0-. () Kingon, A. I.; Srinivasan, S. Lead Zirconate Titanate Thin Films Directly on Copper Electrodes for Ferroelectric, Dielectric and Piezoelectric Applications. Nat. Mater. 0,, -. () Ferren, R. A. Advances in Polymeric Piezoelectric Transducers. Nature, 0,. () Wang, Z. L.; Song, J. H. Piezoelectric Nanogenerators Based on Zinc Oxide

13 ACS Nano Page of 0 Nanowire Arrays. Science 0,, -. () Nguyen, T. D.; Deshmukh, N.; Nagarah, J. M.; Kramer, T.; Purohit, P. K.; Berry, M. J.; McAlpine, M. C. Piezoelectric Nanoribbons for Monitoring Cellular Deformations. Nat. Nanotechnol.,, -. () Michel, K. H.; Verberck, B. Theory of Elastic and Piezoelectric Effects in Two-Dimensional Hexagonal Boron Nitride. Phys. Rev. B 0, 0,. () Duerloo, K. A. N.; Ong, M. T.; Reed, E. J. Intrinsic Piezoelectricity in Two-Dimensional Materials. J. Phys. Chem. Lett.,, -. () Wu, W. Z.; Wang, L.; Li, Y. L.; Zhang, F.; Lin, L.; Niu, S. M.; Chenet, D.; Zhang, X.; Hao, Y. F.; Heinz, T. F.; Hone, J.; Wang, Z. L. Piezoelectricity of Single-Atomic-Layer MoS for Energy Conversion and Piezotronics. Nature,, 0-. () Qi, J.; Lan, Y. W.; Stieg, A. Z.; Chen, J. H.; Zhong, Y. L.; Li, L. J.; Chen, C. D.; Zhang, Y.; Wang, K. L. Piezoelectric Effect in Chemical Vapour Deposition-Grown Atomic-Monolayer Triangular Molybdenum Disulfide Piezotronics. Nat. commun.,,. () Wu, W.; Wang, L.; Yu, R.; Liu, Y.; Wei, S. H.; Hone, J.; Wang, Z. L. Piezophototronic Effect in Single-Atomic-Layer MoS for Strain-Gated Flexible Optoelectronics. Adv. Mater.,, -. () Zhang, X.; Liao, Q.; Liu, S.; Kang, Z.; Zhang, Z.; Du, J.; Li, F.; Zhang, S.; Xiao, J.; Liu, B.; Ou, Y.; Liu, X.; Gu, L.; Zhang, Y. Poly(-styrenesulfonate)-induced sulfur vacancy self-healing strategy for monolayer MoS homojunction photodiode. Nat. Commun.,,. () Zhu, H. Y.; Wang, Y.; Xiao, J.; Liu, M.; Xiong, S. M.; Wong, Z. J.; Ye, Z. L.; Ye, Y.; Yin, X. B.; Zhang, X. Observation of Piezoelectricity in Free-Standing Monolayer MoS. Nat. Nanotechnol., 0, -. () Wang, F.; Seo, J. H.; Luo, G. F.; Starr, M. B.; Li, Z. D.; Geng, D. L.; Yin, X.; Wang, S. Y.; Fraser, D. G.; Morgan, D.; Ma, Z. Q.; Wang, X. D. Nanometre-Thick Single-Crystalline Nanosheets Grown at The Water-Air Interface. Nat. commun.,, 0.

14 Page of ACS Nano 0 () Wang, X.; He, X.; Zhu, H.; Sun, Li.; Fu, W.; Wang, X.; Hoong, L. C.; Wang, H.; Zeng, Q.; Zhao, W.; Wei, J.; Jin, Z.; Shen, Z.; Liu, J.; Zhang, T.; Liu, Z. Subatomic Deformation Driven by Vertical Piezoelectricity From CdS Ultrathin Films. Sci. Adv.,, e0-e0. () Wagner, S. R.; Zhang, P. P. Surfaces and Interfaces of Nanoscale Silicon Materials. MRS Proc., 0,. () Zhao, P. D.; Kiriya, D.; Azcatl, A.; Zhang, C. X.; Tosun, M.; Liu, Y. S.; Hettick, M.; Kang, J. S.; McDonnell, S.; Santosh, K. C.; Guo, J. H.; Cho, K.; Wallace, R. M.; Javey, A. Air Stable P-doping of WSe by Covalent Functionalization. ACS Nano,, () Kalinin, S.V.; Bonnell, D. A. Imaging Mechanism of Piezoresponse Force Microscopy of Ferroelectric Surfaces. Phys. Rev. B 0,,. () Johann, F.; Hoffmann, A.; Soergel, E. Impact of Electrostatic Forces in Contact-Mode Scanning Force Microscopy. Phys. Rev. B 0,, 00. () Scrymgeour, D. A.; Hsu, J. W. P. Correlated Piezoelectric and Electrical Properties in Individual ZnO Nanorods. Nano lett. 0,, -. () Ogawa, T.; Oikawa, H.; Kojima, A. Decrement Caused by Screening Effect of Conduction Electrons on The Effective Charge of CdS Crystals. J. Appl. Phys., 0,. () Zhang, Z.; Liao, Q.; Yu, Y.; Wang, X.; Zhang, Y. Enhanced Photoresponse of ZnO Nanorods-based Self-Powered Photodetector by Piezotronic Interface Engineering. Nano Energy,, -. () Zhang, Y.; Yang, Y.; Gu, Y.; Yan, X.; Liao, Q.; Li, P.; Zhang, Z.; Wang, Z. Performance and Service Behavior in -D Nanostructured Energy Conversion Devices. Nano Energy,, -. () Wang, Z. L. Piezopotential Gated Nanowire Devices: Piezotronics and Piezo-Phototronics. Nano Today 0,, -. () Wang, Z. L.; Wu, W. Piezotronics and Piezo-Phototronics: Fundamentals and Applications. Natl. Sci. Rev.,, -0. () Wu, W.; Wang, Z. L. Piezotronics and Piezo-phototronics for Adaptive

15 ACS Nano Page of 0 electronics and Optoelectronics. Nat. Rev. Mater.,,. () Nemeth, B.; Symes, M. D.; Boulay, A. G.; Busche, C.; Cooper, G. J. T.; Cumming, D. R. S.; Cronin, L. Real-Time Ion-Flux Imaging in The Growth of Micrometer-Scale Structures and Membranes. Adv. Mater.,, -.

16 Page of ACS Nano 0 Figures Figure. Strong Out-of-plane piezoelectricity in ultrathin ZnO material with wurtzite structure for ultrathin piezotronic transistor. a) Ultrathin ZnO materials with wurtzite structure. b) Viewing ZnO structure from the side. c) ZnO nanomaterials with a hexagonal structure consist of Zn-O-Zn stacking with the O atom centered in the tetrahedron (left). A piezoelectric field points along the c-axis when the unit cell of ZnO nanomaterial is compressed by external force in the z direction (right). d) Schematic illustration of ultrathin piezotronic transistor with two-terminal configuration. The stress-induced strong out-of-plane piezopotential in ZnO ultrathin film can effectively modulate the metal-semiconductor barriers, so as to control the carrier transport.

17 ACS Nano Page of 0 Figure. Materials characterization. a) AFM topography scans of typical ultrathin film. Scale bar, µm. b) HRTEM image of the ZnO ultrathin film. Scale bar, nm. c) Electronic property of ZnO ultrathin film demonstrates that the ZnO ultrathin film is p type. d-i) Amplitudes versus applied voltages (.~. V), showing inverse piezoelectricity. The insets are statistical distribution of amplitude values of ZnO ultrathin film and substrate. Scale bars, 0 nm. j) Average amplitude variations versus applied voltages derived from the statistical distributions of amplitude values of ZnO ultrathin film and substrate (d-i). Error bars indicate standard deviations. The piezoelectric coefficient d eff is evaluated to be ~. pm V.

18 Page of ACS Nano 0 Figure. Electrical transport properties of ultrathin piezotronic transistor with M-S-M structure under stress. a) Cross-sectional view of the structure of an ultrathin piezotronic transistor together with atomic force microscopy measurement used to characterize the device. b) The asymmetric modulation of carrier transport by stress in ultrathin piezotronic transistor shows characteristics of piezotronic effect. c) Band diagrams explaining the piezotronic behavior occured in ultrathin piezotronic transistor. φ d and φ s represent the Schottky barrier heights formed at contacts. E p indicates the change of Schottky barrier height by piezoelectric polarization charges. d) Real-time measurement of the current through ultrathin piezotronic transistor that reversibly switches between the closed and open forms upon pressure pulses at a bias of.0 V (left). The current responses between s and s (right).

19 ACS Nano Page of 0 Figure. Stress-gated OR logic gates using series of ultrathin piezotronic transistors. (a,b) OR gate with one force on and the measured output currents for the 0 state. (c,d) OR gate with one force on and the measured output currents for the 0 state. (e,f) OR gate with the both forces on and the measured output currents for the state. The current is much smaller in the 00 state than that in the 0, 0 and states. The current is much larger in the state than that in 0 and 0 states.

20 Page of ACS Nano 0 Abstract Graphic